Conventional golf balls can be divided into two general types or groups: solid balls or wound balls. The difference in play characteristics resulting from these different constructions can be quite significant. These balls, however, have primarily two functional components that make them work. These components are the core and the cover. The primary purpose of the core is to be the “spring” of the ball or the principal source of resiliency. The primary purpose of the cover is to protect the core.
Two-piece solid balls are made with a single-solid core, usually made of a crosslinked polybutadiene or rubber, which is encased by a hard cover material. In these balls, the solid core is the “spring” or source of resiliency. The resiliency of the core can be increased by increasing the crosslink density of the core material. As the resiliency increases, however the compression may also increase making a ball with increased stiffness. Stiffness is a physical attribute defined by load per unit of deflection. In the golf ball art, stiffness is commonly measured using Atti and Rheile “compression” gauges, however, other methods can be used.
Multi-piece solid balls include multi-layer core constructions or multi-layer cover constructions, and combinations thereof. In a golf ball with multi-layer core, the principal source of resiliency is the multi-layer core. In a golf ball with a multi-layer cover, the principal source of resiliency is the single-layer core.
Wound balls, on the other hand, typically have either a solid rubber or fluid-filled center around which many yards of a stretched thread or yarn are wrapped to form a wound core. The wound core is then covered with a durable cover material that adheres to the wound core. Since stretched threads or yarns are extremely resilient, the wound layer acts as the “spring” or source of resiliency for wound balls. The wound balls achieve high resiliency while having a much lower compression than can be achieved with solid cores of similar resiliency. In a wound ball, the center functions primarily as the point about which the winding process begins. The solid center may have some small influence on the ball's overall resiliency, but the principle “spring” of the ball is the wound layer. The fluid in a fluid-filled center can act as a third design element in the ball, but does not substantially contribute to resiliency of the ball.
Attempts have been made in recent years to improve the efficiency of the wound layer by using different winding techniques, different thread materials, or layering the windings. While these solutions have lead to improvements in resiliency or altered the spin of wound balls, none of these improvements alters the fact that the windings act as the “spring” or main source of resiliency in the ball.
Another type of ball has evolved which employs a very large core and a very thin layer of elastic windings that forms a hoop-stress layer. In many golf balls, the ball diameter is about 1.68 inches. In such golf balls with a large core, the core has a diameter of between 1.50 and 1.63 inches. In such golf balls, the thickness of the thin wound layer is between 0.01 and 0.10 inches. In one example, the large core includes a center and a layer of conventional windings subsequently wound with threads that form a hoop-stress layer. U.S. Pat. No. 5,713,801 to Aoyama discloses such a golf ball. The hoop-stress layer aids in rapidly returning the core to its spherical shape, and is a separate layer from the cover or core. The hoop-stress layer has about the same thickness as inner cover layers on many double-cover designs. Though most of the ball's resiliency comes from the core, the contribution of the wound hoop-stress layer to resiliency is significant.
Golf balls with diameters greater than 1.68 inches may be called “oversized” golf balls. In such “oversized” golf balls, the ball diameter can exceed 1.72 inches. Golf balls with diameters significantly less than 1.68 inches may be called “British” golf balls. In such “British” golf balls, the ball diameter can be as small as 1.62 inches. In either “oversized” or “British” golf balls the core can have different sizes, but the thickness of the hoop-stress layer will remain the same as in the large core golf ball (i.e., between 0.01 and 0.10 inches).
As discussed above, the primary purpose of the cover is to protect the “spring.” Different covers vary in the types of protection they provide, and different cores have different protection requirements. Polybutadiene cores in solid balls are adversely affected by moisture, and their covers should have good moisture barrier properties and should be applied to the cores soon after their formation. On the other hand, if a wound core is exposed to air, the windings may oxidize rapidly and lose their resiliency. As a result, wound balls require covers that protect them from oxidation. Additionally, the thread of wound cores should also be prevented from unraveling. Balls with wound hoop-stress layers must be protected from oxidation and unraveling similarly to wound cores. Furthermore, if balls with wound hoop-stress layers have large solid cores, they must also be protected from moisture similarly to solid cores. As a result, the cover of balls with wound hoop-stress layers must be selected with these requirements in mind.
A strong correlation has been observed between the stiffness of the cover and the resiliency of the ball. The stiff or hard ionomer covers can function as a hoop-stress layer providing both core protection and improved resilience. However, the better a cover functions as a hoop-stress layer, the harder it feels and the worse it performs greenside. When a stiff ionomer is used as an inner cover, the inner cover materials although providing hoop-stress typically cannot match the resiliency of a layer of stretched rubber thread (i.e., wound hoop-stress layer) of the same thickness.
Hence, there remains a need for a cover design that will provide improved resilience while also having good abrasion durability, good hardness, and friction characteristics that result in favorable spin.